The invention pertains generally to digital data processing and, more particularly, to methods and apparatus for implementation of declarative rule-based systems, and their integration into mainstream computing environments. The invention has application, by way of non-limiting example, in the design and implementation of workflow applications. Such systems typically have object-oriented architectures, and the invention takes advantage of aspects of such architectures, but it is not limited to these.
The vast majority of data processing systems use a procedural-based programming paradigm. This is exemplified by programming languages like C or Java, where execution is controlled by “procedures” consisting of sequences of statements (assignments, loops, if, etc.). The programmer of such a system is responsible for specifying, in great detail, what this sequence is, and making sure that it is correct. The sequential nature of such systems, and the resulting exponentially large number of execution paths that a system of even moderate size can take, is the fundamental basis for their complexity. It is also the source of most of the very large number of bugs that typically plague such systems.
Declarative rule-based programming is a fundamentally different programming paradigm. It is characterized by the lack of any sequential statements, and any notion of state of which the programmer has to be aware. Instead, processing is specified in the form of logical rules, where each rule relates one or more output variables to a set of input variables. Such rules can be “chained” together so that the output of one rule becomes an input to other rules.
An important characteristic of such a system is that the rules always hold. In other words, the values of the variables are always such that they are consistent with the rules. For example, if a rule states a relation between variables like z=x+y, then the values of x, y, and z are constrained to be such that this relation is never violated. This implies that changes to input variables (x or y in this example) must be propagated “instantly” so that the corresponding output variables are updated according to the relation. For example, if x=1, y=2, and z=3, and then x is changed to 2, then z must be updated to 4. In this case, “instantly” means that the update happens before any user, or any processing element outside the declarative programming system, notices an inconsistency.
The primary advantage of a declarative rule-based system is that it allows the programmer to focus on the required behavior of the application by specifying (declaring) invariant logical and arithmetic relations between variables. The complexity related to sequencing and state is hidden from the programmer.
In many applications, declarative rules can be used for solving significant portions of a problem. However, they are usually not appropriate for every aspect of a typical workflow problem. Some things (a sequence of steps to be executed for a work item, for example) are best expressed procedurally.
In view of the foregoing, an object of this invention is to provide improved methods and apparatus for digital data processing and, more particularly, for integrating declarative and procedural programming systems.
Another object of the invention is to provide such integrated computational systems that are relevant to a wide variety of applications, including, for example, modeling and processing workflows.
A related object of the invention is to provide methods and apparatus for improved integration of declarative and procedural programming systems that are suited for integration with systems having object-oriented data architectures.
A further object is to provide such methods and apparatus as can be implemented in a variety of programming languages and/or on a variety of platforms.